Radioactive reference source for instrument calibration and comparison measurements



A 1 c. CHRISTIANSON ET AL 3,337,735

RADIOACTIVE REFERENCE SOURCE FOR INSTRUMENT CALIBRATION AND COMPARISON MEASUREMENTS Original Filed June 12, 1961 2 Sheets-Sheet 1 2 L I i 3 32 5% I l I w 3%1 QM/@465 C/fg/ E7 g/ 3/ 5%N @9 J EHLPA/ C. Mega/0 4M1 xiw Aug. 22, 1967 c 5T AN N ET AL 3,337,735

RADIOACTIVE REFERENCE SOURCE FOR INSTRUMENT CALIBRATION AND COMPARISON MEASUREMENTS Original Filed June 12, 1961 2 Sheets--Sheet z;

0770 may United States Patent 3,337,735 RADIOACTIVE REFERENCE SOURCE FOR IN- STRUMENT CALIBRATION AND COMPARI- SON MEASUREMENTS Charles Christianson, New York, N.Y., and Ralph C. Maggio, Fort Lee, N.J., assignors to the United States of America as represented by the Secretary of the Navy Original application June 12, 1961, Ser. No. 116,629, now Patent No. 3,217,165, dated Nov. 9, 1965. Divided and this application Sept. 2, 1964, Ser. No. 397,064

2 Claims. (Cl. 250-106) This application is a division of application Ser. No. 116,629, filed June 12, 1961 for Radioactive Reference Source and Method for Making the Same, issued Nov. 9, 1965, Patent Number 3,217,165.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to calibrating radioactivity instruments and making measurements of radioactivity particularly beta activity of samples.

An object of this invention is to provide comparatively inexpensive radioactive sources, particularly beta sources superior to those available heretofore for use in calibration and comparison measurements.

A further object is to provide improved radioactive sources for use in calibrating and making measurements.

FIG. 1 is an exploded view of an embodiment of a radioactive source in accordance with this invention,

FIG. 2 is an assembly view in section of the structure shown in FIG. 1,

FIG. 3 is a view in section of another embodiment of a radioactive source in accordance with this invention,

FIG. 4 is a perspective view of a radioactive source set including sources as shown in FIGS. 1-3,

FIG. 5 illustrates a counting device.

An embodiment of a beta source in accordance with this invention is shown in FIGS. 1 and 2 and includes a circular planchet having a flat bottom 14 and a narrow circumferential rim 16 normal to the bottom of the planchet. A comparatively thin (approx. inch) compact, fiat absorbent pad 18 of substantially the same diameter as the inside diameter of the planchet is mounted within the planchet. The absorbent pad is backed by a stiffening disk 20 of paper, stiff fabric, or the like. One face of the stiffening disk is cemented by an adhesive layer 22 to one face of the absorbent pad and the opposite face of the stiffening disk is cemented by an adhesive layer 22 to the inside face of the bottom of the planchet. The absorbent pad I18 includes a beta source impregnant 26, indicated by reference character but not specifically illustrated, e.g. strontium 90 in the form of strontium chloride. Strontium 90 has a radioactive daughter product yttrium 90 and this is generally designated Sr90-Y90. For complete identification of Sr-90, the percentage of the total disintegration in the strontium 90 and in the yttrium 90, as of a particular date, is specified. Strontium 90 is long lived and its energy level is convenient for calibration and measurement.

The lateral distribution of the beta source impregnant in the pad 18, as distinguished from the depth distribution, is approximately uniform. The depth distribution over the entire pad is approximately the same so that beta emanation from the outwardly directed face of the pad is approximately uniform over that face. In other words, while the density distribution in depth may be shaded from a maximum at one face to a minimum at the opposite face, it is substantially uniform if sensed in plan. A thin film 28 of vinylidene chloride overlays the pad and is secured in place over the pad with a thin-walled plastic ring 30 forced 3,337,735 Patented Aug. 22, 1967 planchet; the marginal circular portion of the film 28 is bonded to the ring 30 or is frictionally gripped between the ring 30 and the rim. The film 28 may be colored as an identification aid. It serves to protect the source against contamination and deterioration through abrasion and protects the user.

When counting equipment is calibrated with a standard source it is necessary to use a standard source having an energy distribution as nearly identical as possible to that of the samples counted by that equipment. The geometry of a counter is strongly dependent upon energy. The National Bureau of Standards makes available standards of some isotopes.

If the source 10 is to be a standard source, the radio active impregnant is precisely identified; the weight of material, the specific activity, the total disintegrations per unit time, and the decay rate, e.g. the disintegrations per unit time on a plurality of future dates are provided with the source. If the radioactive material has a radioactive daughter product on disintegration, the percentage of the total disintegrations from parent and daughter (eg. strontium 90, yttrium as of a particular time are provided; an excellent reference is the date of secular equilibrium, that is when the disintegrations of parent and daughter are equal. If the source is intended as a reference beta source by a technician making drift adjiustments on counting instruments, the beta count as of a particular time under a specified counting geometry and decay date on the impregnant material are needed.

If the source 10 is a radioactive sample to be calibrated by comparison with a standard, the weight of the impregnant sample must be determined accurately and, if available, the identity of the isotope must be specified. If the identity of the isotope in the sample is not known, its identity will be assumed to be the most dangerous or damaging isotope under the circumstances in which the sample was taken; from the biological safety standpoint, the unknown may be calibrated under the assumption that it is strontium 90.

Chemical materials available in chemical supply houses are suitable for fabricating the embodiment shown in FIG. 1. For example, the planchet may be an RCL Model 30301 aluminum sample pan about 11 K inches inside diameter; this pan can retain approximately two cubic centimeters of liquid. The absorbent pad may be a Hollingsworth-Vose No. 5 filter disc of 1 inches diameter. This filter is essentially a flat absorbent cotton pad stiffened by linen gauze on one face and being on the order of inch thick. This pad can be wetted substantially to saturation by about one cubic centimeter of water. The vinylidine chloride may be colored Saran Wrap-5, gauge 50, manufactured by Dow Chemical Company. A considerable variety of soluble radioactive salts particularly water soluble radioactive salts are available commercially; standard or calibrated radioactive salt solutions suitable for use in making a standard source are available from the National Bureau of Standards.

To fabricate the embodiment shown in FIG. 1, the gauze face of the absorbent pad 18 and the stiffening material 20 are cemented together face to face with an interposed thin layer of adhesive 22; the adhesive is applied to the stiffening material only and when tacky the pad and the stiffening material are joined. The adhesive is not applied directly to the pad to avoid impregnating the pad and thereby reducing its absorbency. Then the other face of the stiffening material 20 is cemented by a layer of adhesive 24 to the inside face of the bottom of the planchet. Preferably the adhesive, when dry, is nonwettable by the solvent in the impregnating solution. If the solvent in the impregnating solution is water the adhesive is of a type whose surface is comparatively noninside the rim of the wettable by water, e.g. rubber cement. A predetermined quantity of a radioactive salt solution sufiicient to substantially saturate the pad, e.g. one cubic centimeter is drawn into a micropipette of the type having a plunger type syringe or bulb attached to one end. The planchet is supported horizontally with the exposed face of the pad facing upwardly and the liquid in the micropipette is gradually discharged, droplet by droplet, onto the exposed face of the pad with successive drops distributed about the pad surface to achieve a substantially uniform lateral distribution of the impregnant. The droplets are dispensed gradually to permit complete absorption of each droplet by the pad as the liquid is dispensed so as to avoid flow through or spreading of the liquid along the exposed face to the perimeter or rim of the planchet. By using a non-wetting adhesive, any liquid that passes through the pad and reaches the adhesive is reabsorbed by the pad. The planchet is supported horizontally while the pad air-dries. The uniformity of the lateral distribution of radioactive impregnant is checked with a counter and a shield having a perforation. With the planchet in a fixed geometry in the counter, the shield is gradually moved about to enable radiations from different sectors to be seen by the counter in order to determine whether the radiation intensity from the pad varies significantly along its lateral dimensions. If the radiation intensity is uniform the source is used; if the radiation intensity along the pad varies significantly, the planchet is discarded. If the source is to be a standard source, an accurate count is taken under a specified repeatable counting geometry. Then the exposed face of the pad is covered with a circular film of colored vinylidene chloride 28 and secured in place with a thin-walled plastic ring 30. Then a count is taken under the specified counting geometry. The absorption factor of protective film and ring are calculated from the before and after counts. The embodiment of the invention shown in FIG. 1 has not only in preparing sample sources, standard sources and reference sources but in addition, has utility in the counting of radioactive contamination in atmospheric dust. An absorbent pad 18 is in FIGS. 1 and 2 capable of trapping approximately all the dust in the air passed therethrough, e.g. particles as small as 0.3 micron, is mounted between opposed fine wire gauge supports in an air conduit and a measured volume of air is forced through the pad and discharged at a point sufificiently spaced from the air intake to minimize recycling. After a predetermined volume of air is passed through the pad, the pad is removed and mounted to duplicate the structure shown in FIG. 1. In other words, the planchet, pad, stiffening, and adhesive comprising the standard and the atmospheric dust sample are substantially identical. On the sample the film 23 and ring 3%) may be omitted if the count on the sample is taken without delay. If the contaminant in the atmosphere is known, the standard source is fabricated using the same radioactive material as an impregnant, but generally in monitoring the contaminant is not precisely identified. When the contaminant is not identified, the planchet containing the dust sample is compared with a standard containing the most feared contaminant under the circumstances. Counts are taken of sample and standard source planchets under the same counting geometry. If the sample planchet includes no film 28 nor ring 30, correction is made for their absorption factor in comparing sample and standard. Since the disintegrations per minute of the standard is known, the disintegrations per minute in the dust sample is obtained by equating the ratio of count and disintegrations per minute for each planchet. The disintegration per minute in the sample divided by the volume of air from which the sample was obtained yield the disintegrations per minute per unit volume of air.

While the distribution of material in the two absorbent pads may not be identical and while the dust in the air includes non-radioactive miscellaneous matter which when trapped in the pad has some absorptive effect on the emanations from the radioactive dust, the order of magnitude of inaccuracy is low and generally predictable by the experienced user.

The specific activity of a radioactive salt can be determined by comparison against a standard containing the same salt or against a radioactive salt having a similar energy distribution. A device as shown in FIG. 1 is fabricated using a predetermined quantity of a predetermined percent solution of the salt whose specific activity is to be measured. The steps for making the sample planchet are substantially the same as those for making the standard source and the count of sample and standard is taken in the same geometry.

To obtain the activity of a sample through comparison against a standard, sample and standard are fabricated into sources as described. The sample and standard source are counted under the same geometry. The disintegrations per minute of the standard are known. If the sample includes film 28 and ring 30, the disintegrations per minute are determined by equating the ratios of count and disintegrations per minute of standard and sample. If the sample source does not include the film 23 and ring 30, correction is made in the count of the standard source for absorption by these elements.

The device in FIG. 1 may be used to obtain a beta count from dust settled on a surface. A pad 18 is used for swiping up a wiping of a dust sample, or surface film. The pad is weighed before and after to determine the quantity of material gathered. A sample planchet is made, a count taken and comparison is made against a standard source having a selected impregnant. Though the measurement is somewhat inexact due to the absorptive effect of nonactive material in the sample and the distribution of the sample in the pad, the order of magnitude of error is low and for periodic monitoring for detecting change the error may be ignored.

In FIG. 3 there is shown a source 32 in accordance with this invention including a planchet 34 having a substantially planar inside bottom surface and a peripheral rim normal to the bottom. The inside bottom surface of the planchet is coated with a thin uniform film of radioactive material 36 coated with a protective film of methyl methacrylate.

A method of making a source as shown in FIG. 3 is to deposit in the planchet with a calibrated micropipette, and while the planchet is supported with its bottom surface horizontal, a predetermined quantity of an aqueous solution of a radioactive material and of predetermined concentration. After the solution is in the planchet, a drop of a vaporizable wetting agent, e.g. acetone, is added to the solution and then the contents are dried at a temperature lower than would cause the solution to boil or spatter. The wetting agent prevents uneven distribution of the solid over the bottom inside surface. After the radioactive film is dried, a beta count is taken in a predetermined repeatable geometry and the deposited film of radioactive material is coated with a protective film, e.g. methyl methacrylate. Methyl methacrylate is marketed in spray cans in retail hardware stores; One commercial example is Krylon, crystal clear No. 1301 spray coating. After the protective film is applied, a count is taken again in the same geometry to ascertain the absorption factor of the protective film. The absorption factor for the film is on the order of 3 percent. Therefore, small differences on film thickness in a pair of compared planchets are not significant. This embodiment may be made as a standard source, a reference source or a sample source. If the embodiment is to be a standard source, the precise character of the deposit is identified, controlled, recorded and made available with the source. The radioactive deposit is carefully selected, the weight of the deposit and the disintegrations per unit time are predetermined and the decay rate, e.g., the disintegrations per unit time on a plurality of future dates are provided with the source. If the deposited radioactive material has a radioactive daughter product on disintegrations, the percentage of the disintegrations from parent and daughter (e.g. strontium 90, yttrium 90) respectively, as of a particular time are provided if needed; an excellent reference condition is secular equilibrium, that is when the counts from parent and daughter are equal. If the source is intended for use as a reference beta source by a technician making drift adjustments on counting instruments, the beta count as of a particular time under a specified counting geometry, and decay data on the radioactive material are needed. If the source is a radiocative sample to be calibrated by comparison with a standard, the weight of the sample must be determined accurately and, if available, the identity of the isotope must be specified. If the identity of the isotope in the sample is not known, its identity will be assumed to be the most dangerous or damaging isotope under the circumstances under which the sample was taken.

An advantage of practising the invention using the embodiment in FIG. 3 is that the procedure is faster and simpler, deposits are thinner and there is less absorption. However, the bottom of the planchet must be as fiat as possible to insure even distribution and great care must be exercised to insure that the bottom is horizontal during the time drying takes place. Because some of the material deposited on the inside surface of the rim the source cannot be made as uniform as can a source as illustrated in FIG. 1. The embodiment in FIG. 3 cannot be used in connection with measurements of radioactivity in the atmospheric dust or settled dust.

In making standard sources as in FIG. 1 or FIG. 3, it is advantageous to make a family of similar sources difliering from one another in intensity so that a sample source may be compared against a standard having approximately the same order of magnitude of intensity. In FIG. 4 there is shown a family of devices of the type shown in FIG. 1 and FIG. 3 removably supported in a convenient housing 38 and having activity and decay data thereon.

In FIG. 5 there is shown a counting apparatus 40 with provision for a predetermined repeatable geometry. The

counting mechanism includes a shield housing 42, a 40 Geiger-Muller tube 44, an access door, not shown, mounting brackets 46, a removable shelf 48 shaped or polarized to occupy a specific position on the brackets 46 and having a central hole 50 to seat one of the described sources and in a repeatable counting geometry in the counting apparatus.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practised otherwise than as specifically described.

We claim:

1. A radioactive source comprising a circular planchet having an approximately planar bottom and a comparatively narrow circumferential rim normal to the bottom of the planchet, a thin circular pad of fibrous absorbent material with stiffened backing, said backing side being cemented to the inside bottom of the planchet with rubber cement, said pad including along its. lateral dimensions an approximately uniform density of a known quantity of a known radioactive salt of known specific activity, a thin sheet of colored transparent vinylidine chloride covering said fibrous material to protect and identify the latter, and a thin Walled collar force-fitted inside the rim to retain said vinylidine chloride covering in place over the impregnated pad.

2. A radioactive source comprising a circular planchet having an approximately planar bottom and a comparatively narrow circumferential rim normal to the bottom of the planchet,

a thin circular pad of fibrous absorbent material with stifiened backing,

said backing side being cemented to the inside bottom of the planohet,

said pad including along its lateral dimensions an approximately uniform density of a known quantity of a known radioactive salt of known specific activity, and 1 a protective film of low radioactivity absorptiveness over said pad.

References Cited UNITED STATES PATENTS ARCHIE R. BORCHELT Primary Examiner, 

1. A RADIOACTIVE SOURCE COMPRISING A CIRCULAR PLANCHET HAVING AN APPROXIMATELY PLANAR BOTTOM AND A COMPARATIVELY NARROW CIRCUMFERENTIAL RIM NORMAL TO THE BOTTOM OF THE PLANCHET, A THIN CIRCULAR PAD OF FIRBOUS ABSORBENT MATERIAL WITH STIFFENED BACKING, SAID BACKING SIDE BEING CEMENTED TO THE INSIDE BOTTOM OF THE PLANCHET WITH RUBBER CEMENT, SAID PAD INCLUDING ALONG ITS LATERAL DIMENSIONS AN APPROXIMATELY UNIFORM DENSITY OF A KNOWN QUANTITY OF A KNOWN RADIOACTIVE SALT OF KNOWN QUANTITY THIN SHEET OF COLORED TRANSPARENT VINYLIDINE CHLORIDE COVERING SAID FIBROUS MATERIAL TO PROTECT AND IDENTIFY THE LATTER, AND A THIN WALLED COLLAR FORCE-FITTED INSIDE THE RIM TO RETAIN SAID VINYLIDINE CHLORIDE COVERING IN PLACE OVER THE IMPREGNATED PAD. 